Material Selection and Processing Techniques for Common Mechanical Parts

 

In the field of mechanical design and manufacturing, selecting the appropriate materials and suitable processing techniques is crucial for ensuring the performance, quality, and cost-effectiveness of mechanical parts. This article provides an in-depth analysis of material selection principles, types of materials, and processing techniques for common mechanical parts, aiming to offer valuable insights for mechanical engineers and related professionals.

 

 

I Material Selection Principles

 

The selection of materials for mechanical parts requires a comprehensive consideration of multiple factors, including performance requirements, processability, and economic efficiency. Specifically, the main principles of material selection include the following:

 

1. Performance Requirements

The material must first meet the specific performance requirements under the operating conditions of the part, such as strength, hardness, wear resistance, corrosion resistance, and fatigue resistance. This requires the designer to have a thorough understanding of the working conditions of the part, including the forces it will encounter, the working environment (such as temperature, humidity, and exposure to certain substances), and any special requirements (such as thermal conductivity, electrical conductivity, and magnetic properties).

 

2. Processability

The material's processability directly affects the ease of manufacturing, production efficiency, and cost. Good processability includes factors such as forgeability, weldability, and machinability, which help reduce manufacturing difficulty and cost, and improve production efficiency.

 

3. Economic Efficiency

While meeting performance and processability requirements, it is important to select materials that are as cost-effective as possible to minimize the overall product cost. This requires designers to consider material prices, processing costs, and subsequent maintenance and usage costs comprehensively.

 

 

▲ Steps for Material Selection in Mechanical Parts

 

 

II Common Types of Materials

 

The commonly used materials for mechanical parts mainly include the following categories:

 

1. Steel

Steel is one of the most commonly used materials for mechanical parts, characterized by high strength, good plasticity and toughness, as well as excellent machining performance. Depending on the application and performance requirements, steel can be divided into ordinary carbon structural steel, high-quality carbon structural steel, alloy structural steel, and cast steel, among others.

 

2. Cast Iron

Cast iron has good castability, vibration damping, and wear resistance, but its strength and toughness are relatively low. Therefore, cast iron is often used to manufacture parts that bear static loads and friction wear, such as machine tool beds and housings.

 

3. Non-Ferrous Metals

Non-ferrous metals, such as copper and copper alloys, aluminum and aluminum alloys, have lower density and good electrical and thermal conductivity, making them widely used in electrical, electronic, and aerospace fields. In mechanical parts, non-ferrous metals are often used to manufacture lightweight structural components, sliding bearings, etc.

 

4. Engineering Plastics

With the development of materials science, the application of engineering plastics in mechanical parts is becoming increasingly widespread. Engineering plastics are lightweight, corrosion-resistant, and have good insulation properties, making them suitable for manufacturing non-load-bearing, corrosion-resistant, or insulated parts.

 

 

III Analysis of Machining Processes

 

The machining processes for mechanical parts are diverse, commonly including turning, milling, drilling, grinding, forging, and injection molding. Different machining processes are suitable for different materials and part shapes, which will be introduced separately below.

 

1. Turning

Turning is a method of machining where the workpiece is fixed in a rotating holding device, and tools gradually cut away material from the workpiece to achieve the desired shape and size. Turning is suitable for manufacturing cylindrical parts, such as shafts and sleeves. The accuracy and surface roughness of turning depend on the choice of tool and the setting of cutting parameters.

 

2. Milling

Milling is a method of machining that cuts material from the surface of a workpiece using a rotating tool to manufacture flat surfaces, concave and convex surfaces, gears, and other complex-shaped parts. Milling can be divided into types such as face milling, vertical milling, end milling, gear milling, and contour milling, with each type suitable for different machining needs.

 

3. Drilling

Drilling is a machining method that cuts material from a workpiece using a rotating drill bit to form holes of required diameter and depth. Drilling is commonly used to manufacture hole-type parts, such as bolt holes and bearing holes. The precision and efficiency of drilling depend on the selection of drill bits, the setting of cutting parameters, and the implementation of cooling and lubrication measures.

 

4. Grinding

Grinding is a method of machining that gradually cuts or grinds material from the surface of a workpiece using abrasive tools to achieve the desired shape, size, and surface quality. Grinding is suitable for machining parts with high precision and surface quality requirements, such as molds, precision mechanical parts, and tools. The precision and surface quality of grinding depend on the choice of abrasive tools, the setting of grinding parameters, and the clamping method of the workpiece.

 

5. Forging

Forging is a metal processing method that shapes hot-processed metal materials into desired forms through pressing. Forging is suitable for manufacturing parts with complex shapes and high mechanical performance requirements, such as gears and shafts. Forging can improve the internal organizational structure of materials, enhancing the strength and toughness of parts.

 

6. Injection Molding

Injection molding is a process in which molten plastic is injected into a mold and solidified to produce required parts. Injection molding is suitable for manufacturing large quantities of complex-shaped plastic parts, such as mobile phone cases and automotive components. The precision and surface quality of injection molding depend on the design of the mold, the performance of the injection molding machine, and the choice of plastic materials.

 

 

IV Detailed Design of Typical Bow and Arrow Process Routes

 

Shaft Parts

 

The commonly used materials for shaft parts vary widely, primarily chosen based on factors such as strength, stiffness, wear resistance, manufacturability, and economic considerations. Below are some common materials for shaft parts and their characteristics:

 

1. Carbon Steel

• High-Quality Carbon Structural Steel: Such as grades 35, 45, 50, etc., these steels have high overall mechanical properties and are widely used. Among them, 45 steel is the most commonly used due to its good performance. To further enhance its mechanical properties, normalizing or quenching and tempering treatments are often performed.
• Ordinary Carbon Structural Steel: Such as Q235, Q275, etc., these steels are suitable for shafts that bear relatively low or less critical loads.

 

2. Alloy Steel

Alloy steel has high mechanical properties but is relatively expensive, commonly used for shafts with special requirements. For example, shafts operating under high speed, heavy load, or high-temperature conditions often use alloy steel to meet specific performance requirements. Common alloy steels include 20Cr, 20CrMnTi, 40CrNi, 38CrMoAlA, etc., which, after carburizing, quenching, and other heat treatments, can significantly enhance the wear resistance, impact toughness, and fatigue strength of the shafts.

 

3. Cast Iron

Ductile Iron: Due to its good casting performance, it is easy to cast into complex shapes and has good vibration damping and low stress concentration sensitivity. It is often used to manufacture complex-shaped shafts. Especially, rare earth-magnesium ductile iron developed in China has been widely applied in the manufacturing of important shaft parts in the automotive, tractor, and machine tool industries due to its excellent impact toughness, friction reduction, and vibration absorption properties.

 

4. High-Strength Cast Iron

Also has good casting and vibration damping properties, making it suitable for manufacturing complex-shaped shafts.

 

 ▲ 38CrMoAIA Example

 

 

Gear Parts

 

The selection of gear materials is a complex process that requires consideration of various factors, including load requirements, speed, environmental conditions, noise and vibration requirements, and economic considerations.

 

1. Forged Steel

Forged steel is made through the forging process and has superior properties compared to ordinary steel. Forged steel gears have high strength, high wear resistance, and good impact resistance, making them one of the most commonly used materials for manufacturing gears. Common forged steel materials include 45 steel, which can have its mechanical properties further enhanced through heat treatment.

 

2. Cast Steel

Cast steel is commonly used for manufacturing larger gears because large diameter gears are not suitable for forging. Cast steel gears have good strength and toughness, but their internal structure may not be as dense as forged steel and may require heat treatment to improve their performance.

 

3. Alloy Steel

Alloy steel is made by adding a certain amount of alloying elements to ordinary steel, resulting in higher strength and hardness. Alloy steel gears are often used in applications that bear larger loads and operate at higher speeds.

 

4. Gray Cast Iron

Gray cast iron is relatively brittle and has poor impact resistance and wear resistance, but it has good anti-galling and anti-pitting capabilities. Therefore, gray cast iron gears are commonly used in situations where operation is smooth, speeds are low, and power is not high.

 

5. Ductile Iron

Ductile iron is treated through a special heat treatment process, causing the graphite in its internal structure to distribute in a spherical form, which improves the strength and toughness of the cast iron. Ductile iron gears have good comprehensive performance and are suitable for applications with higher performance requirements.

 

6. Non-Metallic Materials

Plastics, powder metals, etc.: In certain special circumstances, such as light load, low speed, or when noise and vibration reduction is required, non-metallic materials can be used to manufacture gears. These materials are lightweight, produce low noise, and are easy to process, but their load-bearing capacity and wear resistance are inferior to metallic materials.

 

For example, gears used in automobiles or tractors.

 

▲ 20CrMnTi Example

 

Tool Parts

 

The selection of materials for tools is crucial for their performance and lifespan. When choosing tool materials, various factors must be considered, including the tool's purpose, the properties of the materials being processed, cutting conditions, and economic considerations. Below are some common tool materials:

 

1. High-Speed Steel

 

Characteristics:

• High strength and good toughness, allowing for sharp cutting edges.
• Good workability, easy to forge and machine.
• Divided into ordinary high-speed steel and high-performance high-speed steel. Ordinary high-speed steel has lower cutting speeds, while high-performance high-speed steel is produced by increasing carbon and vanadium content and adding elements such as cobalt and aluminum, resulting in higher durability.

 

Applications:

• Generally used for complex-shaped tools, such as twist drills, taps, reamers, gear cutters, and form tools, often made from high-speed steel.

 

2. Cemented Carbide

 

Characteristics:

• Extremely high hardness, with hardness reaching 74–82 HRC at room temperature and heat resistance up to 900–1000°C, offering excellent cutting performance.
• High cutting speeds, capable of exceeding 100 m/min when cutting carbon steel.
• However, it has poor toughness, weak impact and vibration resistance, and edges are not easily sharpened to extreme sharpness, with poorer machinability.

 

Classification and Applications:

• Cemented carbide is classified according to GB2075-87 into P, M, and K types. P-type is used for processing long chips of black metals; M-type is used for processing black metals and non-ferrous metals; K-type is for processing short chips of black metals, non-ferrous metals, and non-metallic materials.

 

3. Stainless Steel

 

Characteristics:

• Easy to maintain, strong corrosion resistance, suitable for use in damp or corrosive environments.
• However, its hardness and sharpness may not be as high as high-carbon steel, and it is usually more expensive.

 

Applications:

• Used in household kitchen knives, medical surgical instruments, and other situations requiring frequent cleaning and hygiene maintenance.

 

4. High-Carbon Steel

 

Characteristics:

• High hardness and good sharpness, suitable for precision machining and cutting.
• However, it is prone to rust and requires regular maintenance and care.

 

Applications:

• Used in razors, outdoor knives, and other applications requiring high hardness and sharpness.

 

 

▲ Gear Hobs

 

 

 ▲ Gear Hobs Main Process Description

 

 

V Conclusion

 

The selection of materials and processing methods for common mechanical parts is a complex and systematic process that requires consideration of multiple factors. When selecting materials, choices should be based on the performance, processing capabilities, and economic aspects of the parts; when choosing processing methods, decisions should be made according to the shape, size, surface requirements of the parts, and the characteristics of the materials. Through rational selection of materials and processing techniques, the performance, quality, and cost-effectiveness of mechanical parts can be optimized.